1. Field of the Invention
The present invention is directed to a method and magnetic resonance system for registering magnetic resonance signals of a subject and reconstructing an image on the basis of the registered magnetic resonance signals.
2. Description of the Prior Art
Methods of the above type are known wherein an operator provides a control device with an input (entry) for the magnetic resonance system which designates a reconstruction region in a phase-coding direction within which the image should be reconstructed on the basis of the magnetic resonance signals to be registered, and wherein the control device effects the registration of the magnetic resonance signals in the phase-coding direction in the reconstruction region and beyond the reconstruction region as well in two supplemental regions that are disposed at both sides of the reconstruction region in the phase-coding direction and which immediately adjoin said reconstruction region.
The spatial resolution or coding in magnetic resonance tomography usually ensues in three steps.
When a magnetic resonance excitation signal is transmitted, first, a gradient field is superimposed on a uniform basic magnetic field in an excitation direction. The magnetic resonance signal therefore only excites elements to magnetic resonance that are disposed in a plane perpendicular to the excitation direction. The overall informational signal that is received later arises from these excited elements.
Upon reception of the magnetic resonance signals, a gradient field is likewise superimposed on the basic magnetic field. This gradient field, however, is oriented in a reception direction that differs from the excitation direction. Generally, the reception direction is perpendicular to the excitation direction. Due to the superimposition of this gradient field, the excited spins oscillate with a frequency that is dependent on the position of the excited spins within the plane excited by the reception gradient field. A further resolution in an axial direction onto a single axis is thus possible by means a later frequency analysis of the received magnetic resonance signal.
In order to also be able to distinguish individual points (volume elements, voxels) from one another within the magnetic resonance signals thus obtained, a short-duration gradient pulse is superimposed on the basic magnetic field between the emission of the magnetic resonance excitation signal and the reception of the magnetic resonance signal. This pulse is superimposed in a further direction that is linearly independent of, and usually perpendicaular to, the excitation direction and the reception direction. The various locations in this direction, which is usually referred to as the phase-coding direction, differ on the basis of the phase of the spins. The number of voxels that can be resolved in phase-coding direction thereby corresponds to the number of magnetic resonance signals registered with the various phase codings (referred to as raw data rows). Due to the fact that a phase coding can be exactly defined to a maximum of 2xcfx80 (360xc2x0), the maximum phase coding in the phase-coding direction therefore is only allowed to be 360xc2x0.
Often, only a part of the acquirable measurement volume is of significance in phase-coding direction. For example, a viewer may be interested only in the left pulmonary wing of a patient. It is therefore known to provide the control device, as input thereto, with the reconstruction region of interest within which the image should be reconstructed.
A number of versions for the registration of the magnetic resonance signals are known.
First, there is the possibility of registering magnetic resonance signals from the entire measurement region that can be covered in the phase-coding direction. This, however, requires the registration of many raw data rows. The patient thus must spend a relatively long time in the magnetic resonance tomography apparatus, which is often considered extremely unpleasant by patients. Moreover, very large datasets, which must be processed, arise given this procedure.
Of course, it would be possible to register a lower number of raw data rows of the total measurement region, however, a low resolution that is often inadequate would then occur.
It has also been proposed to register raw data rows only from the reconstruction region and to reference the maximum phase of 2xcfx80 to this region. In this case, however, it may occur that artifacts in the reconstructed image are generated due to subjects lying outside the reconstruction region but inside the covered measurement region. These artifacts cannot be avoided due to the fact that the phase offset can only be exactly defined within 2xcfx80.
It is therefore usual for the control device to effect the registration of the magnetic resonance signals in the phase-coding direction beyond the reconstruction region as well in two supplemental regions that are disposed at both sides of the reconstruction region in the phase-coding direction and which immediately adjoin said reconstruction region. The two supplemental regions conventionally are of the same size and they are defined such that no elements that could elicit disturbing artifacts are outside the supplemental regions. In this case, the maximum phase coding of 2xcfx80 refers to the boundaries of the supplemental regions. Generally, the operator provides (designates) these supplemental regions by an input (entry) into the control device. This procedure already assures a more or less efficient operation of the magnetic resonance system. In particular, a partial optimization of the number of required raw data rows already occurs in order to enable an adequately high-resolution reconstruction in the reconstruction region while simultaneously avoiding artifacts.
An object of the present invention is to provide a method and apparatus of the above type that represent an over conventional methods and systems.
This object is achieved in a magnetic resonance system and an operating method therefor of the above type wherein the supplemental regions can be selected independently of one another.
The size of the supplemental regions thus can be optimized independently of the position of the reconstruction region relative to the subject to be acquired. This is possible in conventional methods only in the case of an accidentally symmetrical position of the reconstruction region relative to the subject to be acquired.
As warranted, the supplemental regions can be defined automatically by the control device. This, for example, can ensue by means of a test scan in a single plane or a single line. However, it is simpler when the operator prescribes the supplemental regions for the control device.